Public safety communication systems and private/commercial land mobile systems deliver both data and voice to their subscribers. Such public safety communication and private land mobile communications systems can utilize various technologies to deliver the data and voice. In general, narrowband radio links are utilized for this purpose. More recently, Public Safety systems often utilize broadband methods to deliver increasing content at faster rates. Examples of such technologies include Land Mobile Radio (LMR) systems, Citizens Broadband Radio Service (CBRS), and Long Term Evolution (LTE) cellular systems.
Subscribers utilize mobile devices to send and receive data over these communication systems. Mobile devices can be less useful when they are only allowed to access one of the various over the air technologies, such as LMR, CBRS, or LTE. To solve this problem, mobile devices have been manufactured and programmed to have the ability to access multiple technologies. For example, mobile devices exist to that can utilize both LMR and LTE systems, thereby making the mobile devices more useful, more fault tolerant, and having increased geographic scope.
A talkgroup call is a push to talk (PTT) communication including all members who are registered with a talkgroup. One problem with such dual-mode devices is that they can experience what is called “the lunchroom effect” during talkgroup calls. In LMR systems, a user depresses a button on a mobile device, and the audio provided by that user is transmitted to all other talkgroup members In a narrowband system, this is typically done by directing all talkgroup callers to a base station that is carrying the talkgroup call over a single radio frequency channel. The resultant transmission is directed to all recipients in broadcast mode; all listeners to the message receive it simultaneously. So if multiple subscribers, such as public safety officers, are in the same location, they will all be receiving the same radio signal over the same frequency at the same time, thereby alleviating issues of multiple, diverse delayed reception to each subscriber. Such delayed reception of individual audio streams would result in multiple streams exhibiting random delayed audio output from the broadcast subscriber device, thereby causing significant disruption and distraction to the group of users if they are co-located. In the single channel, broadcast condition mode, they will all be receiving and playing the audio on their mobile devices at virtually the same time. The effect at the multi-user reception site is similar to having multiple devices listening to a single FM radio station.
However, dual-mode device may receive the talkgroup call and corresponding audio on an LTE or other unicast, broadband channel. In a unicast system, each subscriber, therefore each recipient, receives audio through a separate effective virtual channel. This LTE channel would be dedicated to this receiving talkgroup mobile device. When a single device is located in a particular location, the user will hear the audio, and if the audio is delayed for a small amount of time there is no deleterious effect.
However, if multiple talkgroup members are located in the same location and are all in LTE mode, each will be receiving the talkgroup audio on a different channel. Because each channel will have different amounts of delay, each device will receive the audio at a different time. As more and more dual-mode subscribers are located in a single location and using LTE to receive the talkgroup audio, the delay in audio on each device will lead to the audio being played at different times. This will make the audio very difficult to understand, and will quickly annoy all users ate the location, as “the lunchroom effect” has audio playing at slightly different times.
Therefore a need exists for a way of allowing dual-mode or multi-mode mobile devices to participate in talkgroup calls without playing audio at a different timing as other talkgroup users located at the same location.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.